The present invention relates to layered, absorbent structures constituted by fibrous layers with interposed particles of hydrogelling absorbent material.
The structures may be used as absorbent elements in disposable absorbent articles such as absorbent articles for incontinent adults, babies' nappies, sanitary towels, dressings and the like.
Disposable absorbent articles are well known and all have absorbent elements for absorbing and retailing body fluids; an absorbent element must be able to acquire liquid rapidly and to distribute it internally so as to prevent leakages and must also have a good capacity to retain the fluids when subjected to the normal pressures of use.
Absorbent elements made mainly of hydrophilic fibrous material such as, for example, pads of cellulose fibres, layers of wadding, or the like generally have satisfactory characteristics as regards their liquid-absorption rate and can distribute the liquid effectively within them but are very ineffective from the point of view of retention when subjected to the normal pressures of use.
The use of hydrogelling absorbent materials in combination with hydrophilic fibres in order to increase the absorption and retention capacities of the absorbent elements is known.
Hydrogelling absorbent materials, commonly known as superabsorbents, are polymers which can swell up and absorb large quantities of liquid, particularly water, or also, to a lesser extent, body fluids.
They also have the particular property that they retain the fluids even under moderate pressure; owing to this characteristic, their use in absorbent elements for disposable absorbent articles has been proposed for some time.
The high absorption capacity of superabsorbents is not, however, combined with similarly rapid absorption and this may adversely affect the performance of absorbent articles incorporating these substances.
In fact, superabsorbents may give rise to a phenomenon defined in the prior art as "gel blocking"; when a particle of superabsorbent is in contact with the liquid, its external surface starts to absorb the liquid and swells up obstructing the transmission of the liquid into the particle itself; the liquid can penetrate further into the still-dry core of the particle only by means of a very slow diffusion mechanism.
This phenomenon may prevent full use being made of the large absorption capacities of superabsorbent substances.
Leaving out of consideration the type of superabsorbent material and the shapes and sizes of the particles, "gel blocking" is generally encouraged by the formation of agglomerations of superabsorbent particles within the absorbent structure, which may occur either before use, when the particles are still dry, or during use, when the particles start to absorb the liquid and swell up.
In any case, with the use of hydrogelling absorbent materials, it is possible to produce absorbent elements which contain less hydrophilic fibres for a given absorption capacity and which consequently have smaller dimensions, particularly thicknesses, than conventional absorbent elements made of fibres alone.
Structures have been formed in which the fibres and the particles of hydrogelling, absorbent material are disposed in separate, generally very thin, superposed layers.
Many particular forms of layered, absorbent structures of this type, in which the fibrous material is represented by one or more layers of wadding, absorbent paper or non-woven fabric, and in which the particles of hydrogelling absorbent material are incorporated in the structure in various ways, are known in the art.
In layered structures which are formed dry, the at least two fibrous layers are bound together solely by the intertwining of fibres and the particles of hydrogelling absorbent material between them are held in position by the two fibrous layers joined together; a better connection between the fibrous layers can be achieved by subjecting the structure to an embossing process which encourages the fibres to intertwine by causing slight relative slippage between the layers, as described in U.S. Pat. No. 4,578,068.
Alternatively, the layered structure may be formed damp by the application of water, or preferably steam, and pressure; the connection between the fibrous layers is achieved mainly by virtue of the particles of hydrogelling, absorbent material which become sticky and act as an adhesive between the fibrous layers.
In neither case is the structure very stable and the fibrous layers may separate, for example, along their edges, leaving the particles of hydrogelling absorbent material free to slide between the surfaces of the layers between which they are included; the particles of hydrogelling absorbent material may thus become concentrated locally thereby encouraging, and possibly aggravating, the establishment of the "gel blocking" phenomenon in use, and may also escape from the edges of the structure, in any case reducing the absorption capacity of the structure.
Another solution for the formation of a layered absorbent structure provides for the use of an adhesive, for example, of the hot melting type, applied to the surface of one of the fibrous layers with the dual purpose of bonding the two fibrous layers together and simultaneously fixing the particles of superabsorbent material between them.
The use of an adhesive may, however, affect the absorption characteristics both of the fibrous layer to which the adhesive is applied, and of the superabsorbent particles which come into contact with the adhesive.
In general, therefore, it is necessary not to use an excessive quantity of adhesive and consequently not to eliminate completely the possibility of the loss of superabsorbent material from the edges of the layered structure.
For this reason, layered absorbent structures of the type described are formed directly on the production line for the absorbent articles in which they are to be incorporated, since it is not convenient to produce them independently as semi-finished products.
The problem of the loss of hydrogelling absorbent material along the edges is common to all the layered structures described up to now and has been solved in various ways in known products; for example, by surrounding the structure completely with a layer of wadding, which involves the addition of a further element, or by the use of a single layer of fibrous material on which the adhesive and the superabsorbent material are distributed only on a central longitudinal strip and subsequently folding the two side portions so that they partially overlap approximately on the longitudinal axis.
This solution solves the problem of the loss of hydrogelling absorbent material from the edges of the structure without requiring the presence of additional elements but does not allow different materials to be used for the two fibrous layers. Moreover, it involves the use of a larger quantity of material than would be needed simply to superpose two layers of the same width as the finished product.
There remains therefore the problem of providing a thin, layered, absorbent structure which does not have the disadvantages described above connected with the method of joining together the various layers of which it is made up and, preferably, of reducing the quantity of material so as to produce a thinner and lighter structure.
The invention described in International Patent Application No PCT/US93/06128 filed Jun. 25, 1993 and claiming priority from Italian Patent Application No TO92 A 000566 filed Jul. 3, 1992 has the object of improving the absorbtion and stability characteristics of thin, layered, absorbent structures containing hydrogelling, absorbent material for use in disposable absorbent articles.
According to the said invention this object was achieved by virtue of a thin, layered, absorbent structure having the specific characteristics recited in the associated claims.
A further subject of that invention was a method for the manufacture of such a structure.
In summary, that invention related to a thin, layered, absorbent structure formed by at least two fibrous layers comprising between them a layer of particles of hydrogelling, absorbent material, the two fibrous layers being joined together by particles of thermoplastic, polymeric, organic material distributed and mixed with the hydrogelling, absorbent material, and by two lines of adhesive disposed along the longitudinal edges of the structure.
In application PCT/US93/06128, we stated that the quantity of the hydrogelling absorbent material (hereinafter sometimes referred to by its usual abbreviation "AGM") together with the thermoplastic polymeric organic material may be between 30 g/m.sup.2 and 150 g/m.sup.2.
We further stated that the quantity of the latter material should be between 5 g/m.sup.2 and 30 g/m.sup.2.
Given that the quantity of the latter material needs to increase as the quantity of AGM increases, the values given above are to be understood as implying that the maximum quantity of AGM is 120 g/m.sup.2.
There was indeed a good reason why such a maximum should exist. Photomicrographic analysis of the structure according to PCT/US93/06128 showed that the two fibrous layers were held together by "bridges" between them, formed by a particle of the thermoplastic material which had fused to both fibrous layers, or a plurality of such particles which had fused to one another and, at the ends of the bridge, to the fibrous layers.
Increasing the quantity of AGM would evidently reduce the probability of such bridges being formed, and a point would be reached (and was reached by the time the quantity of AGM had increased to the upper limit of 120 g/m.sup.2) where the bridges were too few in number to provide a stable structure.